In recent years, various light-emitting polymer compounds have been developed as optical devices of organic light-emitting devices etc. used for organic electroluminescence (EL) devices, light-emitting phototransformation devices or wavelength-transformation devices, etc.
If a polymer compound is soluble in organic solvents, it can be formed into a film by a wet process such as spin coating method, dipping method or ink jet method, etc., and it is easy to adjust the film thickness when forming a film.
When producing an optical device, the film formation by a wet process using such polymer compound lowers the optical device production cost, as compared with a vapor deposition process which requires a vacuum unit when forming a metal thin film or a low molecular weight organic thin film. Therefore, there is a demand for a light-emitting polymer compound that has heat resistance and strength, is soluble in organic solvents and usable for optical devices, and emits fluorescence.
As such light-emitting polymer compound, π-conjugated polymer compounds, such as poly-p-phenylene and polyphenylene vinylene, are generally known. Such π-conjugated polymer compounds are easily soluble in organic solvents, but have had a problem that heat resistance and a long term stability of fluorescence characteristics, such as fluorescence strength and fluorescence spectrum, are not sufficient.
Polyimides and polyamides have superior heat resistance, strength and electrical characteristics, but are low in fluorescence characteristics, in contrast with the above-mentioned light-emitting polymer compounds.
For example, as described in Non-patent Publication 1, polyimides are known to emit only a weak fluorescence against the ultraviolet irradiation and therefore to be extremely low in luminous efficiency. Index of luminous efficiency is expressed by light emission quantum yield defined as the proportion of the number of photons emitted by fluorescence to the number of photons absorbed into fluorescent molecules by excitation. In entire aromatic polyimides, light emission quantum yield is less than 0.01%. A very low luminous efficiency of polyimides is caused by a charge-transfer complex formed between an electron-donating diamine moiety and an electron-accepting acid anhydride moiety in the molecular structure. As mentioned in Non-patent Publication 2, it is known that a polyimide obtained by a polycondensation between pyromellitic anhydride as a typical polyimide and 4,4′-diaminophenyl ether emits only a very weak fluorescence of 9.7×10−7% in fluorescence quantum yield, which is difficult to be measured by an ordinary fluorescence spectrometer.
Thus, various molecular designs have been made in order to improve luminous efficiency of polyimides.
For example, in Patent Publication 1 and Patent Publication 2, there is disclosed a fluorescent material formed of an independent film or a nanofiber aggregate. It is said to be able to synthesize a polyimide having a high light emission quantum yield of around 10% by using an alicyclic diamine as a raw material to provide steric hindrance, thereby eliminating an electron-donating diamine moiety to prevent an intermolecular charge transfer among polyimides.
The polyimide described in Patent Publication 1 and Non-patent Publication 3 is, however, a polyimide using an alicyclic diamine or an alicyclic acid dianhydride as a raw material and on the other hand is a semi-alicyclic polyimide combined with an aromatic ring. Therefore, there has been a problem of being difficult to say that it is superior in heat resistance and strength. Furthermore, in entire aromatic polyimides superior in heat resistance and strength, there has been a problem of no achievement of high light-emission characteristics.
There is conducted research of introducing a side chain having a chemical structure showing fluorescence, in order to obtain light emission property in polyimides.
For example, in Non-patent Publication 4, there is described a polyimide showing a blue color light emission by introducing a fluorescent furyl group into a main chain or side chain. In Patent Publication 2, there is disclosed an organic EL device characterized by having used a polymer thin film having at least one of light emission layer function or charge transfer function or charge injection function, and a polyimide is used in the polymer thin film. In an organic light emission device having a light emission layer and a charge transfer layer, the organic light emission device disclosed in Patent Publication 3 is such that the charge transfer layer is made of a particular polymer, and a polyimide is used as the polymer.
However, fluorescence emission of the polyimides described in Non-patent Publication 4, Patent Publication 2 and Patent Publication 3 results from a fluorescent substituent introduced into a main chain or a side chain of the polyimide, and light intensity of the polyimide obtained by introducing a fluorescent substituent is formed by a strong charge transfer of an imide moiety and a fluorescent substituent. However, there has been a problem that it is very low as compared with fluorescent strength of a low molecular weight compound having the same fluorescent substituent as that of the polyimide obtained by introducing a fluorescent substituent.
In the case of using polyimide for a device, there is a problem that its dissolution in organic solvents is difficult. A polyimide is obtained by conducting a polymerization reaction between tetracarboxylic dianhydride and diamine in a polar solvent to generate a polyamic acid soluble in the polar solvent and then dehydrating the polyamic acid by a chemical dehydration treatment such as heating treatment or using a dehydration agent to achieve an imidization. Polyamic acid as a precursor is easily formed into a film and easily worked. When making a polyimide after a film formation under a condition of polyamic acid, there has been a problem that chemical change of the polyamic acid as a component prior to the imidization does not occur evenly, resulting in a tendency to make a defective film.
Therefore, it is desirable to form a film under a condition of polyimide. Thus, there is a demand for a polyimide of a structure high in solubility.
In order to provide a polyimide with solubility in organic solvents, it is effective to introduce a polar group such as hydroxy group, carboxyl group or amino group. In Patent Publications 4-6, there are disclosed as a fluorine-containing polymerizable monomer an aromatic diamine containing a hexafluoroisopropanol group having a hydroxy group, and a polymer compound using the same, for example, an aromatic polyimide. The aromatic polyimide is said to be easily soluble in organic solvents. However, there has been no report on aromatic polyimides superior in fluorescence that is a property of emitting fluorescence.